In the field of image projection of a rectilinear object to a proportionately enlarged or reduced rectilinear image (as represented by conventional photographic enlargers and slide projectors), the entire image is projected typically upon a single plane (e.g., in the enlarger, to the photographic paper; and from the slide projector, to the screen). A more difficult task arises when an image must be projected into a display device having two separate image surfaces for the vertical and horizontal components of the image; each of which requires independent magnification and focus of the vertical and of the horizontal image components. The problem is further complicated when one of the image surfaces is tilted with respect to the projection axis; the tilt being so significant that conventional image focus will not be sustained along the full image surfaces. The two disparate image surfaces must be illuminated in such a manner that the vertical and the horizontal image components maintain independent focus along their respective tilted surfaces. Further, since projected images generally expand (or enlarge) over progressively greater projected field distances, tilted image surfaces are also subject to "keystoning", whereby one dimension (say, the horizontal "width") is enlarged progressively more as viewed from the "top" or the "bottom" of the image.
An example of a device which requires such image handling is represented in U.S. Pat. No. 5,381,502 entitled, "Flat or Curved Thin Optical Display Panel". FIG. 1 illustrates the type of panel construction described in the '502 patent. The panel comprises a stack of thin waveguide-like transparent lamina 111 each of typical thickness t. When the stack is cut at an acute angle S, each lamination exhibits a height h at the display surface such that h=t sec S. Thus, with S measuring typically about 70.degree., h is significantly larger than t. Also, the full display height H is larger than the base thickness T by the same factor, sec S.
The device of the '502 patent is called a "polyplanar optic display" (POD). The rightmost portion of the POD is represented primarily in FIG. 1 as an isometric view. The full width W is typically wider than its display height H. The portion which is detailed serves to describe the operation of the POD and is useful in understanding its relationship to the present invention. Each lamination (of thickness t) of the panel is a transparent sheet (glass or plastic) of nominal optical index of refraction n.sub.1, separated by thin coatings of index of refraction n.sub.2, where n.sub.1 &gt;n.sub.2. Light entering the laminations at the base is separated into sheets and is confined to its respective sheets by total internal reflection at the interfaces. Thus, light focused at the base will retain "vertical" resolution elements of thickness t (in the "T"-direction) throughout its propagation "upward" to the display surface, where each thickness t is displayed as a corresponding resolvable height h. In the width W direction, however, there is no confinement of the input illumination, and each sheet propagates its respective slice (in the width direction) as would a continuous transparent medium. This requires that the horizontal image components be focused over varying distances corresponding to the tipped viewing surface. While the vertical component of the projected image must focus near the base, the horizontal information must focus near the sloping plane of the display surface; those components at the "bottom" of the display focusing close to the base, and those higher focusing at progressively greater distances to represent image elements approaching the top of the display. Also, while propagating through the lamina, the horizontal components expand progressively as an extension to the expanding illuminating field. Unless corrected, this generates keystoning, whereby (in this example) the top of the displayed image becomes wider than that at the bottom.
It is among the objects of the present invention to solve image handling problems of the type described above and also to provide image projection that can be used in conjunction with a POD type of display panel.